EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS

Transcription

1 of 3 EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS THE GREEN PLANTS 55 EMBRYOPHYTA- LAND PLANTS 59 DIVERSITY OF NONVASCULAR LAND PLANTS 62 Liverworts 62 Mosses 65 Hornworts 69 Polysporangiophytes/Pan-Tracheophyta 70 REVIEW QjESTIONS 71 EXERCISES 72 REFERENCES FOR FURTHER STUDY 72 THE GREEN PLANTS The green plants, formally called the Viridiplantae or Chiorobionta, are a monophyletic group of eukaryotic organ isms that includes what have traditionally been called green algae plus the land plants or embryophytes (Figure 3.1). Like all eukaryotes, the Viridiplantae have cells with membrane-bound organelles, including a nucleus (containing chromosomes composed of linear chains of DNA bound to proteins, that are sorted during cell division by mitosis), micro tubules, mitochondria, an endoplasmic reticulum, vesicles, and golgi bodies. Although the interrelationships of the non land plant Viridiplantae will not be covered in detail here, it is important to realize that some of the evolutionary innova tions, or apomorphies, that we normally associate with land plants actually arose before plants colonized the land. Several apomorphies unite the Viridiplantae (Figure 3.1). One possible novelty for this group is a cellulosic cell wall (Figure 3.2A). Cellulose, like starch, is a polysaccharide, but one in which the glucose sugar units are bonded in the beta-l,4 position (=3-1,4-g1ucopyranoside). This slight change in chemical bond position results in a very different molecule. Cellulose is secreted outside the plasma membrane as micro scopic fiber-like units called microfibrils that are further intertwined into larger fibril units, forming a supportive meshwork. The function of cellulose is to impart rigidity to the cells, acting as a sort of cellular exoskeleton. The evolu tion of a cellulosic cell wall was a preamble to the further evolution of more complex types of growth, particularly of self-supporting shoot systems. It is not clear if a cellulosic cell wall constitutes an apomorphy for the Viridiplantae alone, as it may have evolved much earlier, constituting an apomorphy for the Viridiplantae plus one or more other groups; in any case, its adaptive significance seems clear. Perhaps the primary apomorphy for the Viridiplantae is a specialized type of chloroplast (Figure 3.2). As discussed in Chapter 1, chloroplasts are one of the major defining charac teristics of traditionally defined plants ; their adaptive sig nificance as organelles functioning in photosynthesis, the conversion - light energy to chemical energy, is unques tioned. Chloroplasts in the Viridiplantae, the green plants, differ from those of most other organisms, such as the red and brown algae, in (1) containing chlorophyll b in addition to chloro phyll a, the former of which acts as an accessory pigment in light capture; (2) having thylakoids, the chlorophyll-containing membranes, that are stacked into grana, which are pancakelike aggregations (see Figure 3.2B,C); and (3) manufacturing as a storage product true starch, a polymer of glucose sugar units (= polysaccharide) in which the glucose molecules are chemically bonded in the alpha-1,4 position (ci-l,4- glucopyranoside). Thus, all green plants, from filamentous green algae in a pond or tide pooi to giant sequoia or Elsevier Inc. All rights reserved. doi: /B

2 - -i.thylakoid i : Green Algae (a paraphyletic group) IE fi true starch storage compound parenchyma antheridium sporophyte/embryo (alternation of generations) cuticle = Embryophytes Land Plants Charophytes Plants Streptophytes Chiorophytes 1i Viridiplantae [Chiorobionta] Green reinhardtii, a unicellular green alga, showing granum of chioroplast. (Photo courtesy of Rick Bizzoco.) chloroplasts. B. Diagram of chloroplast structure of green plants, showing thylakoids and grana. C. Electron micrograph of Chlamydomonas FIGURE 3.2 A. Elodea, whole leaf in face view, showing apomorphies of the Viridiplantae: a cellulosic cell wall and green plant -7:- : granum..- - and Mishler et al. (1994). Important apomorphies discussed in the text are listed beside thick hash marks. FIGURE 3.1 Cladogram of the green plants (Viridiplantae or Chlorobionta), modified from Bremer (1985), Mishler and Churchill (1985), cellulose in cell wall (may have evolved earlier & thus not a synapomorphy for Chlorobionta alone) thylakoids stacked in grana Unique green plant chloroplast features chlorophyll b (chlorophyll a is ancestral) archegonium 56 CHAPTER 3 EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS

3 (2n) (2n) A meiosis Zygote B Zygote fertilization meiosis // fertilization (n) (n) (n) (n) (n) Spores (Adult) Gamete Gamete Spores Egg Sperm Isogamy mitosis Multicelled Stage HAPLOID (n) quotation marks) and are defined as the primarily aquatic plant streptophytes. Green algae occur in a tremendous (Figure 3.3A) with or without flagella, thalloid forms (Figure 3.3B), motile and nonmotile colonies (Figure 3.3C), and nonmotile filaments (Figure 3.3D). Many have flagellated motile are a paraphyletic group (which is why the name is placed in or Streptophyceae (Figure 3.1). The traditional green algae the intracellular cohabitation of an independently living, uni were modified from those that evolved via endosymbiosis, The Viridiplantae as a whole are classified as two sister Viridiplantae, consisting of all chiorophytes and the non land data imply that chioroplasts found in the green plants today cellular prokaryote inside a eukaryotic cell (see Chapter 1). groups: chiorophytes, or Chlorophyceae, and streptophytes, variety of morphological forms. These include single cells ing what is termed a haplontic (or haplobiontic ) life cycle spores, each of which may germinate and develop into a new is free-living, then divides by meiosis to form four haploid ing in a diploid (2n) zygote (Figure 3.4A). The zygote, which gametes that are isomorphic, that is, that look identical. Within the streptophyte lineage that gave rise to the land The primitive type of green plant sexual reproduction (Figure 3.4A). plants, a few innovations evolved that may have been soil (or even on snow!) or in other terrestrial but moist algae inhabit fresh and marine waters and some live in or on Fertilization occurs by union of two of these gametes, result Eucalyptus trees have this same type of chloroplast. Recent cells in at least one phase of their life history. Green habitats. haploid individual, which produces more gametes, complet Rick Bizzoco.) B. Ulva, a thalloid form. C. Volvox, a colonial form. D. Spirogyra, a filamentous form. Above: vegetative form, with large, strains and nonmotile zygotes. spiral chloroplasts. Below: reproductive conjugation stage, showing + and FIGURE 3.3 Examples of non land plant Viridiplantae. A. Chiarnydornonas reinhardtii, a unicellular form. (Photo courtesy of 1/ Oogamy HAPLONTIC UNIT II EVOLUTIONANDDIVERSITYOFPLANT5 57 FIGURE 3.4 Haplontic life cycles in some of the green plants. A. Isogamy. B. Oogamy. HAPLONTIC // Multicelled Stage HAPLOH) (n) VI seems to have been the production of flagellate, haploid (n) mating

4 58 CHAPTER 3 EVOLUTION AND DIVERSiTY OF GREEN AND LAND PLANTS preadaptations to survival on land. First of these was the evolution of oogamy, a type of sexual reproduction in which one gamete, the egg, becomes larger and nonflagellate; the other gamete is, by default, called a sperm cell (Figure 3.4B). Oogamy is found in all land plants but independently evolved in many other groups, including many other algae and in the animals. Several other apomorphies of and within the Viridiplantae include ultrastructural specializations of flagella and some features of biochemistry. Although these have been valuable in elucidating phylogenetic relationships, their adaptive significance is unclear, and they will not be considered further here. An apomorphy for the charophytes, a dade within the streptophytes that includes Coleochaete (Figure 3.5B), Charales (Figure 3.5C E), and the land plants (Figure 3.1), are plasmodesmata. Plasmodesmata are essentially pores in the primary (10) cell wall through which membranes traverse between cells, allowing for transfer of compounds between cells (Figure 3.5A). Plasmodesmata may function in more efficient or rapid transport of solutes, including regulatory and growth-mediating compounds, such as hormones. Members of the Charales, such as the genera Chara and Nitella, are perhaps the closest living relatives to the land plants. These fresh water, aquatic organisms have a haplontic life cycle, and consist of a central axis bearing whorls of lat eral branches (Figure 3.5D) or (if small) leaves on the hap bid body. Some Charales are capable of precipitating calcium carbonate as an outer layer of the plant body (accounting for the common names brittleworts or stoneworts ). Members of the Charales grow by means of a single apical cell, similar to that of some land plants and representing a possible syna pomorphy with them. However, the Charales differ from land plants in lacking true parenchyma (see later discussion). The Charales have specialized male and female gametangia, termed antheridia and oogonia (Figure 3.5C,D). The oogonia are distinctive in having a spirally arranged group of outer tube cells (Figure 3.5D); fossilized casts of oogonia retain the outline of these tube cells (Figure 3.5E). Oogonia and antheridia of the Charales resemble the archegonia and FIGURE 3.5 A. Diagram of plasmodesmata in cellulosic cell wall, an apomorphy of some green plants, including the land plants. B. Coleochaete sp., a close relative to the embryophytes. (Photo courtesy of Linda Graham.) C E. Charales. C. Nitella sp., oogonia and antheridia. D. Chara sp., oogonium and antheridium. Note spiral tube cells of oogonia. E. Tectochara helicteres, a fossil oogonium from the Eocene, showing remnants of spiral tube cells.

5 UNIT II EVOLUTION AND DIVERSITY OF PLANTS Embryophyta land plants Pan-Tracheophytal Polysporangiophytes - t t Tracheophytes vascular plants gametophytic leaves pseudo-elaters in sporangium gametophyte leafy (in some) elaters in sporangium columella in sporangium sporophyte branched with multiple sporangia oil bodies sporophyte photosynthetic, nutritionally independent archegonium antheridium parenchyma cuticle stomates sporophyte/embryo (alternation of generations) aerial sporophyte axis t = extinct FIGURE 3.6 One hypothesis of relationships of the land plants (Embryophyta), with major apomorphies indicated. After Qiu et al. (2007), some apomorphies after Bremer (1985); Mishler and Churchill (1985); Mishler et al. (1994). antheridia of land plants (see later discussion) in having an outer layer of sterile cells, but the gametangia of the two groups are generally thought not to be directly homologous because of major differences in structure and development. However, members of the Charales retain the egg and zygote (although the latter only briefly) on the plant body. This retention of egg and zygote on the haploid body may repre sent a transition to their permanent retention on the gameto phyte of land plants (see later discussion). EMBRYOPHYTA- LAND PLANTS The Embryophyta, or embryophytes (commonly known as land plants), are a monophyletic assemblage within the green plants (Figures 3.1, 3.6). The first colonization of plants on land during the Silurian period, ca. 400 million years ago, was concomitant with the evolution of several important fea tures. These shared, evolutionary novelties (Figure 3.6) con stituted major adaptations that enabled formerly aquatic green plants to survive and reproduce in the absence of a sur rounding water medium. One major innovation of land plants was the evolution of the embryo and sporophyte (Figure 3.6). The sporophyte is a separate diploid (2n) phase in the life cycle of all land plants. The corresponding haploid, gamete-producing part of the life cycle is the gametophyte. The life cycle of land plants, having both a haploid gametophyte and a diploid sporophyte, is an example of a haplodiplontic (also called diplobiontic ) life cycle, commonly called alternation of generations (Figure 3.7). Note that alternation of generations does not necessarily mean that the two phases occur at differ ent points in time; at any given time, both phases may occur in a population. The sporophyte can be viewed as forming from the zygote by the delay of meiosis and spore production. Instead of mei osis, the zygote undergoes numerous mitotic divisions, which result in the development of a separate entity. The embryo is defined as an immature sporophyte that is attached to or sur rounded by the gametophyte. In many land plants, such as the

6 60 CHAPTER 3 EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS HAPLODIPLONTIC LIFE CYCLE ( Alternation of Generations ) z mitosis, growth, & differentiation Embryo /% mitosis, growth, & dfferenuation Sporophyte Body mitosis, growth, & differentiation Sporangium initosis, growth, & differentiation Zygote SPOROPHYTE GENERATION (2N) Sporocyte (Sperm nonflagellate in Conifers, Gnetales, and Angiosperms) fertilization Egg Sperm ) GAMETOPHYTE GENERATION (N) rneiosis lost by reduction and modificationf Archegonium Antheridium in the Angiosperms and some Gnetales mitosis, growth, & differentiation Gametophyte Body / Spores mitosis, growth, & differentiation FIGURE 3.7 Haplodiplontic alternation of generations in the land plants (embryophytes). seed plants, the embryo will remain dormant for a period of time and will begin growth only after the proper environmen tal conditions are met. As the embryo grows into a mature sporophyte, a portion of the sporophyte differentiates as the spore-producing region. This spore-producing region of the sporophyte is called the sporangium. The sporangium is enveloped by a sporangial wall, which consists of one or more layers of sterile, non-spore-producing cells. A sporan gium contains sporogenous tissue, which matures into sporo cytes, the cells that undergo meiosis. Each sporocyte produces, by meiosis, four haploid spores (Figure 3.7). One adaptive advantage of a sporophyte generation as a separate phase of the life cycle is the large increase in spore production. In the absence of a sporophyte, a single zygote (the result of fertilization of egg and sperm) will produce four spores. The elaboration of the zygote into a sporophyte and sporangium can result in the production of literally millions of spores, a potentially tremendous advantage in reproductive output and increased genetic variation. Another possible adaptive value of the sporophyte is associated with its diploid ploidy level. The fact that a sporo phyte has two copies of each gene may give this diploid phase an increased fitness in either of two ways: (1) by potentially pre ventiiig the expression of recessive, deleterious alleles (which, in the sporophyte, may be shielded by dominant alleles, but which, in the gametophyte, would always be expressed); and (2) by permitting increased genetic variability in the sporophyte generation (via genetic recombination from two parents ) upon which natural selection acts, thus increasing the potential for evolutionary change. A second innovation in land plants was the evolution of cutin and the cuticle (Figure 3.8). A cuticle is a protective layer that is secreted to the outside of the cells of the epider mis (Gr. epi, upon + derma, skin ), the outermost layer of land plant organs. The epidermis functions to provide FIGURE 3.8 cuticle cell wall epidernial cell The cuticle, an apombrphy for the land plants.

7 UNIT II EVOLUTION AND DIVERSITY OF PLANTS 61 mechanical protection of inner tissue and to inhibit water loss. The cuticle consists of a thin, homogeneous, transparent layer of cutin, a polymer of fatty acids, and functions as a sealant, preventing excess water loss. Cutin also impregnates the outer cellulosic cell walls of epidermal cells; these are known as a cutinized cell wall. The adaptive advantage of cutin and the cuticle is obvious: prevention of desiccation outside the ancestral water medium. In fact, plants that are adapted to very dry environments will often have a particu larly thick cuticle (as in Figure 3.8) to inhibit water loss. A third apomorphy for the land plants was the evolution of parenchyma tissue (Figure 3.9). All land plants grow by means of rapid cell divisions at the apex of the stem, shoot, and thallus or (in most vascular plants) of the root. This region of actively dividing cells is the apical meristem. The apical meristem of liverworts, hornworts, and mosses (discussed later), and of the monilophytes (see Chapter 4) have a single apical cell (Figure 3.9), probably the ancestral condition for the land plants. In all land plants the cells derived from the apical meristem region form a solid mass of tissue known as parenchyma (Gr. para, beside + enchyma, an infusion ; in reference to a concept that parenchyma infuses or fills up space beside and between the other cells). Parenchyma tissue consists of cells that most resemble the unspecialized, undif ferentiated cells of actively dividing meristematic tissue. Structurally, parenchyma cells (1) are elongate to isodiamethc; (2) have a primary (1 ) cell wall only (rarely a secondary wall); and (3) are living at maturity and potentially capable of continued cell divisions. Parenchyma cells function in single apical cell FIGURE 3.9 Equisetum shoot apex, showing parenchymatous growth form, from an apical meristem. metabolic activities such as respiration, photosynthesis, lat eral transport, storage, and regeneration/wound healing. Parenchyma cells may further differentiate into other special ized cell types. It is not clear if the evolution of both apical growth and true parenchyma is an apomorphy for the land plants alone, as shown here (Figure 3.6). Both may be inter preted to occur in certain closely related green plants, includ ing the Charales. Correlated with the evolution of parenchyma may have been the evolution of a middle lamella in land plants. The middle lamella is a pectic-rich layer that develops between the primary cell walls of adjacent cells (Figure 3.5A). Its function is to bind adjacent cells together, perhaps a prerequisite to the evolution of solid masses of parenchyma tissue. Another evolutionary innovation for the land plants was the antheridium (Figure 3. ba). The antheridium is a type of specialized gametangium of the haploid (n) gametophyte, one that contains the sperm-producing cells. It is distin guished from similar structures in the Viridiplantae in being surrounded by a layer of sterile cells, the antheridial wall. The evolution of the surrounding layer of sterile wall cells, which is often called a sterile jacket layer, was probably adaptive in protecting the developing sperm cells from desic cation. In all of the nonseed land plants, the sperm cells are released from the antheridium into the external environment and must swim to the egg in a thin film of water. Thus, a wet environment is needed for fertilization to be effected in the nonseed plants, a vestige of their aquatic ancestry. Members of the Charales also have a structure termed an antheridium, which has an outer layer of sterile cells (Figure 3.5C,D). However, because of its differing anatomy, the Charales antheridium may not be homologous with that of the land plants, and thus may have evolved independently. Another land plant innovation was the evolution of the archegonium, a specialized female gametangium (Figure 3. lob). The archegonium consists of an outer layer of sterile cells, termed the venter, that immediately surround the egg, plus others that extend outward as a tube-like neck. The archegonium is stalked in some taxa; in others the egg is rather deeply embedded in the parent gametophyte. The egg cell is located inside and at the base of the archegonium. Immediately above the egg is a second cell, called the ventral canal cell, and above this and within the neck region there may be several neck canal cells. The archegonium may have several adaptive functions. It may serve to protect the developing egg. It may also function in fertilization. Before fertilization occurs, the neck canal cells and ventral canal cell break down and are secreted from the terminal pore of the neck itself; the chemical compounds released function as an

8 62 CHAPTER 3 EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS antheridial wall (sterile jacket layer) neck sperm cells neck A B / FIGURE 3.10 A. Antheridia. B. Archegonia. Both are apomorphies of land plants. attractant, acting as a homing device for the swimming sperm. Sperm cells enter the neck of the archegonium and fertilize the egg cell to form a diploid (2n) zygote. In addition to effecting fertilization, the archegonium serves as a site for embryo/sporophyte development and the establishment of a nutritional dependence of the sporophyte upon gametophytic tissue. The land plants share other possible apomorphies: the presence of various ultrastructural modifications of the sperm cells, flavonoid chemical compounds, and a proliferation of heat shock proteins. These are not discussed here. DIVERSITY OF NONVASCULAR LAND PLANTS During the early evolution of land plants, three major, monophyletic lineages diverged before the vascular plants (discussed in Chapter 4). These lineages may collectively be called the nonvascular land plants or bryophytes and include the liverworts, mosses, and hornworts. Bryophytes are a paraphyletic group, defined by the absence of derived features; the name, placed in quotation marks, is no longer formally recognized. Liverworts, mosses, and hornworts differ from the vascular plants in lacking true vascular tissue and in having the tophyte as the dominant, photosynthetic, persistent, and freeliving phase of the life cycle. It is likely that the ancestral gametophyte of the land plants was thalloid in nature, similar to that of the hornworts and many liverworts. The sporophyte game of the liverworts, mosses, and hornworts is relatively small, ephemeral, and attached to and nutritionally dependent upon the gametophyte (see later discussion). The relationships of the liverworts, mosses, and hornworts to one another and to the vascular plants remain unclear. Many different relationships among the three lineages have been proposed, one recent of which is seen in Figure 3.6. LIVERWORTS Liverworts, also traditionally called the Hepaticae, are one of the monophyletic groups that are descendents of some of the first land plants. Today, liverworts are relatively minor ponents of the land plant flora, growing mostly in moist, shaded areas (although some are adapted to periodically dry, hot habitats). Among the apomorphies of liverworts are (1) distinctive oil bodies and (2) specialized structures called elaters, elongate, nonsporogenous cells with spiral wall enings, found inside the sporangium. Elaters are hygroscopic, meaning that they change shape and move in response to changes in moisture content. Elaters function in spore dispersal; as the sporangium dries out, the elaters twist out of the capsule, carrying spores with them (Figures 3.11, 3.12K). There are two basic morphological types of liverwort gametophytes: thalloid and leafy (Figures ). Thalloid liverworts consist of a a flattened mass of tissue; this is likely the ancestral form, based on cladistic studies. As in hornworts and mosses, the gametophyte bears uniseriate, filamentous processes that function in anchorage and absorption. Pores in the upper surface of the rhizoids, thallus, com thick